In recent years, in the fields of optical communication or optical measurement, various optical elements for controlling the intensity or phase of light waves are used. In an optical fiber communication or optical measurement system, optical elements having optical waveguides are used for necessary optical processing from the viewpoint of ease of integration, high control efficiency, and the like. Of these, an optical element having a Mach-Zehnder type optical waveguide is excellent in high-speed capability and stability of the modulation operation of light waves.
In an optical element which uses a substrate having an electro-optical effect, such as lithium niobate, the substrate itself is formed of a thin plate having a thickness equal to or smaller than 50 μm so as to realize the wide bandwidth of the drive frequency of the optical element or to reduce the drive voltage of the optical element. The thinned substrate has weak mechanical strength, so a support substrate is usually bonded to the rear surface of the substrate through an adhesive layer (see Patent Document 1).    Patent Document 1: JP-A-2002-337274
Meanwhile, in an optical element using an optical waveguide, when the manufactured optical element may not achieve desired characteristics due to manufacturing errors or the like, or when the characteristics of the optical element are actively changed, the phase of light waves propagating inside the optical waveguide is regulated. A method of regulating the phase of light waves basically regulates the refractive index of the optical waveguide itself or its periphery (a region which reacts with the propagating light waves). Specifically, there are a method of forming a groove on a substrate as described in Patent Document 2, a method using a photorefractive effect or a photo-elastic effect, a method of forming, on an optical waveguide, a material having a refractive index different from the optical waveguide, and the like.    Patent Document 2: JP-A-11-271552
However, in the existing phase regulation method, a complex and expensive regulation device is necessary at the time of regulation, and after regulation has been done, it is difficult to restore the regulated state to the original state and to regulate the phase of light waves again. For this reason, the regulation operation may be further complicated and may become troublesome.